© 2013 pearson education, inc. figure 11.16 synapses. axodendritic synapses dendrites cell body...
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© 2013 Pearson Education, Inc.
Figure 11.16 Synapses.
Axodendriticsynapses
Dendrites
Cell body
Axoaxonalsynapses
Axon
Axosomaticsynapses
Axon
Axosomaticsynapses
Cell body (soma)of postsynaptic neuron
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Varieties of Synapses: Electrical Synapses
• Less common than chemical– Electrically coupled (joined by gap junctions
that connect cytoplasm of adjacent neurons)• Very rapid• Unidirectional or bidirectional• Synchronize activity
– More abundant in:• Embryonic nervous tissue
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Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynapticneuron
Action potentialarrives at axonterminal.
Mitochondrion
Axon terminal
Synapticcleft
Synapticvesicles
Postsynapticneuron
Postsynapticneuron
Presynapticneuron
1
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Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynapticneuron
Action potentialarrives at axonterminal.
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Mitochondrion
Axon terminal
Synapticcleft
Synapticvesicles
Postsynapticneuron
Postsynapticneuron
Presynapticneuron
1
2
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Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynapticneuron
Action potentialarrives at axonterminal.
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Ca2+ entrycauses synapticvesicles to releaseneurotransmitterby exocytosis
Mitochondrion
Axon terminal
Synapticcleft
Synapticvesicles
Postsynapticneuron
Postsynapticneuron
Presynapticneuron
1
2
3
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Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynapticneuron
Action potentialarrives at axonterminal.
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Ca2+ entrycauses synapticvesicles to releaseneurotransmitterby exocytosis
Neurotransmitter diffusesacross the synaptic cleft andbinds to specific receptors onthe postsynaptic membrane.
Mitochondrion
Axon terminal
Synapticcleft
Synapticvesicles
Postsynapticneuron
Postsynapticneuron
Presynapticneuron
1
2
3
4
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Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Ion movement
Graded potential
Binding of neurotransmitter opension channels, resulting in gradedpotentials.
5
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Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Enzymaticdegradation
Diffusion awayfrom synapse
Neurotransmitter effects areterminated by reuptake throughtransport proteins, enzymaticdegradation, or diffusion awayfrom the synapse.
Reuptake
6
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Synaptic Delay
• Time for NT to be released, diffuse across synapse, and bind to receptors– 0.3–5.0 ms
• Synaptic delay is rate-limiting step of neural transmission
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Postsynaptic Potentials
• NT receptors cause graded potentials that vary in strength with:– Amount of NT released– Time nt stays in area
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Table 11.2 Comparison of Graded Potentials and Action Potentials (1 of 4)
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Table 11.2 Comparison of Graded Potentials and Action Potentials (2 of 4)
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Table 11.2 Comparison of Graded Potentials and Action Potentials (3 of 4)
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Table 11.2 Comparison of Graded Potentials and Action Potentials (4 of 4)
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Postsynaptic Potentials
• Types of postsynaptic potentials – EPSP—excitatory postsynaptic potentials – IPSP—inhibitory postsynaptic potentials
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Excitatory Synapses and EPSPs
• NT binding opens chemically gated channels• Allows simultaneous flow of Na+ and K+ in opposite
directions
• Na+ influx greater than K+ efflux net depolarization called EPSP (not AP)
• EPSP help trigger AP if EPSP is of threshold strength– Can spread to axon hillock, trigger opening of
voltage-gated channels, and cause AP to be generated
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Figure 11.18a Postsynaptic potentials can be excitatory or inhibitory.
An EPSP is a localdepolarization of the postsynaptic membranethat brings the neuroncloser to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing Na+ and K+ to pass through simultaneously.
Threshold
Stimulus
+30
0
–55
–70
Time (ms)10 20 30
Mem
bra
ne p
ote
nti
al (m
V)
Excitatory postsynaptic potential (EPSP)
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Inhibitory Synapses and IPSPs
• Reduces postsynaptic neuron's ability to produce an action potential– Makes membrane more permeable to K+ or
Cl–• If K+ channels open, it moves out of cell• If Cl- channels open, it moves into cell
– Therefore neurotransmitter hyperpolarizes cell• Inner surface of membrane becomes more
negative• AP less likely to be generated
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Figure 11.18b Postsynaptic potentials can be excitatory or inhibitory.
Threshold
Stimulus
+30
0
–55
–70
Time (ms)10 20 30
Mem
bra
ne p
ote
nti
al (m
V) An IPSP is a local
hyperpolarization of the postsynaptic membranethat drives the neuronaway from AP threshold. Neurotransmitter binding opens K+ or Cl– channels.
Inhibitory postsynaptic potential (IPSP)
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Synaptic Integration: Summation
• A single EPSP cannot induce an AP• EPSPs can summate to influence
postsynaptic neuron• IPSPs can also summate • Most neurons receive both excitatory and
inhibitory inputs from thousands of other neurons– Only if EPSP's predominate and bring to
threshold AP
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Two Types of Summation
• Temporal summation– One or more presynaptic neurons transmit
impulses in rapid-fire order• Spatial summation
– Postsynaptic neuron stimulated simultaneously by large number of terminals at same time
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Figure 11.19a Neural integration of EPSPs and IPSPs.
Threshold of axon ofpostsynaptic neuron
Resting potential
0
Mem
bra
ne p
ote
nti
al (m
V)
–55–70
E1 E1
E1
No summation:2 stimuli separated in time cause EPSPs that do notadd together.
Excitatory synapse 1 (E1)
Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
Time
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Figure 11.19b Neural integration of EPSPs and IPSPs.
Threshold of axon ofpostsynaptic neuronResting
potential
0
Mem
bra
ne p
ote
nti
al (m
V)
–55–70
E1
Excitatory synapse 1 (E1)
Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
Time
E1 E1
Temporal summation:2 excitatory stimuli closein time cause EPSPsthat add together.
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Figure 11.19c Neural integration of EPSPs and IPSPs.
Threshold of axon ofpostsynaptic neuron
Resting potential
0
Mem
bra
ne p
ote
nti
al (m
V)
–55–70
E1
Excitatory synapse 1 (E1)
Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
Time
E1 + E2
Spatial summation:2 simultaneous stimuli atdifferent locations causeEPSPs that add together.
E2
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Figure 11.19d Neural integration of EPSPs and IPSPs.
Threshold of axon ofpostsynaptic neuron
Resting potential
0M
em
bra
ne p
ote
nti
al (m
V)
–55–70
E1
Excitatory synapse 1 (E1)
Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
Time
Spatial summation ofEPSPs and IPSPs:Changes in membane potentialcan cancel each other out.
l1
E1 + l1l1
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Integration: Synaptic Potentiation
• Repeated synapse increases ability of presynaptic cell to excite postsynaptic neuron– Ca2+ conc increases in presynaptic terminal
and postsynaptic neuron• Brief high-frequency stimulation partially
depolarizes postsynaptic neuron– Chemically gated channels allow Ca2+ entry– Ca2+ promotes more effective responses to
subsequent stimuli
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Integration: Presynaptic Inhibition
• Excitatory nt release by one neuron inhibited by another neuron via an axoaxonic synapse
• Less neurotransmitter released• Smaller EPSPs formed
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Neurotransmitters
• Language of nervous system• 50 or more nt have been identified• Most neurons make two or more nts
– Neurons can exert several influences• Usually released at different stimulation
frequencies• Classified by chemical structure and by
function
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Classification of Neurotransmitters:Chemical Structure
• Acetylcholine (ACh)– First identified; best understood– Released at neuromuscular junctions ,by
some ANS neurons, by some CNS neurons– Synthesized from acetic and choline by
enzyme choline acetyltransferase– Degraded by enzyme acetylcholinesterase
(AChE)
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Classification of Neurotransmitters: Chemical Structure • Biogenic amines
• Catecholamines– Dopamine, norepinephrine (NE), and epinephrine– Synthesized from amino acid tyrosine
• Indolamines– Serotonin and histamine– Serotonin synthesized from amino acid tryptophan;
histamine synthesized from amino acid histidine
• Broadly distributed in brain– Play roles in emotional behaviors and biological clock
• Some ANS motor neurons (especially NE)• Imbalances associated with mental illness
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Classification of Neurotransmitters:Chemical Structure
• Amino acids• Glutamate• Aspartate• Glycine• GABA—gamma ()-aminobutyric acid
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Classification of Neurotransmitters:Chemical Structure
• Peptides (neuropeptides)• Substance P
– Mediator of pain signals
• Endorphins– Beta endorphin, dynorphin and enkephalins– Act as natural opiates; reduce pain perception
• Gut-brain peptides– Somatostatin and cholecystokinin
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Classification of Neurotransmitters:Chemical Structure
• Purines – ATP!– Adenosine
• Potent inhibitor in brain• Caffeine blocks adenosine receptors
– Act in both CNS and PNS– Produce fast or slow responses– Induce Ca2+ influx in astrocytes
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Classification of Neurotransmitters:Chemical Structure • Gases and lipids - gasotransmitters
• Nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide gases (H2S)
• Bind with G protein–coupled receptors in the brain• Lipid soluble• Synthesized on demand • NO involved in learning and formation of new
memories; brain damage in stroke patients, smooth muscle relaxation in intestine
• H2s acts directly on ion channels to alter function
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Classification of Neurotransmitters:Chemical Structure
– Endocannabinoids• Act at same receptors as THC (active ingredient in
marijuana)– Most common G protein-linked receptors in brain
• Lipid soluble• Synthesized on demand• Believed involved in learning and memory• May be involved in neuronal development,
controlling appetite, and suppressing nausea
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Classification of Neurotransmitters:Function
• Great diversity of functions• Can classify by
– Effects – excitatory versus inhibitory– Actions – direct versus indirect
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Classification of Neurotransmitters:Function
• Effects - excitatory versus inhibitory– Neurotransmitter effects can be excitatory
(depolarizing) and/or inhibitory (hyperpolarizing)
– Effect determined by receptor to which it binds• GABA and glycine usually inhibitory• Glutamate usually excitatory• Acetylcholine and NE bind to at least two receptor
types with opposite effects– ACh excitatory at neuromuscular junctions in skeletal
muscle– ACh inhibitory in cardiac muscle
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Classification of Neurotransmitters: Direct versus Indirect Actions
• Direct action – Neurotransmitter binds to and opens ion
channels– Promotes rapid responses by altering
membrane potential – Examples: ACh and amino acids
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Classification of Neurotransmitters: Direct versus Indirect Actions
• Indirect action – NT acts through second messengers, usually
G protein pathways– Broader, longer-lasting effects similar to
hormones– Biogenic amines, neuropeptides, and
dissolved gases
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Figure 11.21 Indirect neurotransmitter receptor mechanism: G protein-inked receptors.
2
1
3 4
5a
5b
5c
Ligand (1st messenger)
Receptor G protein Enzyme 2ndmessenger
Recall from Chapter 3 that G protein signaling mechanisms are like a molecular relay race.
Neurotransmitter (1st messenger) binds and activates receptor.
Receptor
G protein
Adenylate cyclase Closed ion channel Open ion channel
GDP
Receptor activates G protein.
G protein activates adenylate cyclase.
Adenylate cyclase converts ATP to cAMP (2nd messenger).
Active enzyme
cAMP changes membrane permeability by opening or closing ion channels.
cAMP activates enzymes.
cAMP activates specific genes.
Nucleus
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Channel-Linked (Ionotropic) Receptors: Mechanism of Action
• Ligand-gated ion channels• Action is immediate and brief• Excitatory receptors are channels for small
cations– Na+ influx contributes most to depolarization
• Inhibitory receptors allow Cl– influx that causes hyperpolarization
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Basic Concepts of Neural Integration
• Neurons function in groups• Groups contribute to broader neural
functions• There are billions of neurons in CNS
– Must be integration so the individual parts fuse to make a smoothly operating whole
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Organization of Neurons: Neuronal Pools
• Functional groups of neurons– Integrate incoming information received from
receptors or other neuronal pools– Forward processed information to other destinations
• Simple neuronal pool– Single presynaptic fiber branches and synapses with
several neurons in pool– Discharge zone—neurons most closely associated
with incoming fiber– Facilitated zone—neurons farther away from
incoming fiber
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Presynaptic(input) fiber
Facilitated zone Discharge zone Facilitated zone
Figure 11.22 Simple neuronal pool.
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Types of Circuits
• Circuits– Patterns of synaptic connections in neuronal
pools• Four types of circuits
– Diverging– Converging– Reverberating– Parallel after-discharge
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Figure 11.23a Types of circuits in neuronal pools.
Input Diverging circuit• One input, many outputs• An amplifying circuit• Example: A single neuron in the brain can activate 100 or more motor neurons in the spinal cord and thousands of skeletal muscle fibers
Many outputs
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Figure 11.23b Types of circuits in neuronal pools.
Converging circuit• Many inputs, one output• A concentrating circuit• Example: Different sensory stimuli can all elicit the same memory
Input 1
Input 2 Input 3
Output
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Figure 11.23c Types of circuits in neuronal pools.
Reverberating circuit• Signal travels through a chain of neurons, each feeding back to previous neurons• An oscillating circuit• Controls rhythmic activity• Example: Involved in breathing, sleep-wake cycle, and repetitive motor activities such as walking
Output
Input
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Figure 11.23d Types of circuits in neuronal pools.
Output
Input Parallel after-discharge circuit• Signal stimulates neurons arranged in parallel arrays that eventually converge on a single output cell• Impulses reach output cell at different times, causing a burst of impulses called an after-discharge• Example: May be involved in exacting mental processes such as mathematical calculations
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Patterns of Neural Processing: Serial Processing• Input travels along one pathway to a
specific destination• System works in all-or-none manner to
produce specific, anticipated response• Example – spinal reflexes
– Rapid, automatic responses to stimuli– Particular stimulus always causes same
response– Occur over pathways called reflex arcs
• Five components: receptor, sensory neuron, CNS integration center, motor neuron, effector
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Figure 11.24 A simple reflex arc.
Interneuron
Spinal cord (CNS)
Stimulus
Receptor
Sensory neuron
Integration center
Motor neuron
Effector
Response
1
2
3
4
5
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Patterns of Neural Processing: Parallel Processing
• Input travels along several pathways• Different parts of circuitry deal
simultaneously with the information– One stimulus promotes numerous responses
• Important for higher-level mental functioning
• Example: a sensed smell may remind one of an odor and any associated experiences
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Developmental Aspects of Neurons
• Nervous system originates from neural tube and neural crest formed from ectoderm
• The neural tube becomes CNS– Neuroepithelial cells of neural tube proliferate
to form number of cells needed for development
– Neuroblasts become amitotic and migrate– Neuroblasts sprout axons to connect with
targets and become neurons
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Axonal Growth: Finding the Target
• Growth cone at tip of axon interacts via:– Cell surface adhesion proteins provide anchor points– Neurotropins that attract or repel the growth cone– Nerve growth factor (NGF) keeps neuroblast alive
• Must find target and right place to form synapse– Astrocytes provide physical support
• About 2/3 of neurons die before birth– If do not form synapse with target– Many cells also die due to apoptosis
(programmed cell death) during development
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Figure 11.25 A neuronal growth cone.